Density measuring apparatus



March 28, 1944. J. F. LUHRS 2,345,272

DENSITY MEASURiNG APPARATUS Filed April 6. 1940 4 Sheets-Sheet 1 INVENT OR.

L/OHN f. L u/ms March 28," 1944. F, LQHRS 2,345,272

DENSITY MEASURING APPARATUS Filed April 6 1940 4 Sheets-Sheet 2 INVENTOR JOHN FLU/19s a WWW f ATTORNE March 28, 1944. J. F. LUHRS DENSITY MEASURING APPARATUS Filed April 6,- 1940 4 Sheets-Sheet s INVENTOR Jo/7w F Lamas BY 014 A0\ 2 ATTORNE March 28,1944. J, F. LUHRS DENSITY MEASURING APPARATUS 4 Sheets-Sheet 4 Filed April- 6, 1940 JOHN F. LUHRS FIG. 4.

Gttorncg Patented Mar. 28, 1944 T @FFICE DENSITY MEASURING APPARATUS John F. Luhrs, Cleveland Heights, Ohio, assignor to Bailey Meter Company, a corporation of Delaware Application April 1940, Serial N5. 328,186

' 5 Claims. (01. 265-44) This invention relates to the art or measurin and/or controlling the magnitude of a variable quantity, condition, relation, etc., and particularly such a variable condition as the density of a liquid-vapor mixture.

I have chosen to illustrate and describe as a. preferred embodiment of my invention its adaptation to the measuring and controlling of the density and other characteristics of a flowing heated fluid stream, such as the flow of hydrocarbon oil through a cracking still.

While a partially satisfactory control of the cracking operation may be had from a knowledge of the temperature, pressure and rate of flow of the fluid stream being treated, yet a knowledge of the density of the flowing stream at different points in its path is of a considerably greater value to the operator, but was not available prior to the discovery of Robert L. Rude, as claimed in his Patent No. 2,217,634.

In the treatment of water below the critical pressure, as in a vapor generator, a knowledge of temperature, pressure and rate of flow may be suflicient for proper control, inasmuch a definite tables have been established for interrelation between temperature and pressure and from which tables the density of the liquid or vapor may be determined. However, there are no available tables for mixtures of liquid and vapor.

In the processing of a fluid, such as a petroleum hydrocarbon, a change in density of the fluid may occur through at least three causes.

1. The generation or formation of vapor of the liquid, whether or not separation. from the liquid occurs.

2. Liberation of dissolved or entrained gass.

3. Molecular rearrangement as by cracking or polymerization.

the density of such a flowing stream is of ,tre-

mendous importance and value to an operator in controlling the heating, mean density, time of detention and/ or treatment in a given portion of the circuit, etc. A continuous knowledge of the density of such a heated flowing stream is particularly advantageous where wide changes in density occur due to formation, generation, and/or liberation of gases, with a resulting formation of liquid-vapor mixtures, velocity changes, and varying time of detention in 'difierent portions of the fluid path. In fact, for a fixed or given volume of path, a determination of mean density in that portion provides the only possi bility of accurately determining the time that the fluid in that portion of the path is subjected to heating or treatment. By my invention I provide the requisite system and apparatus wherein such information is made available continuously to an operator and furthermore comprises the guiding mean for automatic control of the process or treatment.

While illustrating and describing my invention as preferably adapted to the cracking of petroleum hydrocarbon, it is to be understood that it may be equally adaptable to the vaporization or treatment of other liquids and in other proc esses.

In the drawings:

Fig. 1 is a diagrammatic arrangement of the invention in connection with a heated fluid stream.

Fig. 2 is similar to Fig. l but, while diagrammatic, is in greater detail than Fig. 1.

Fig. 3 is a diagrammatic illustration of a further embodiment of my invention.

Fig. 4 is a diagrammatic illustration-of a further embodiment of my invention.

Referring now in particular to Fig. 1, I indicate a conduit I which may be considered as comprising the once through fluid path of an oil still wherein a portion of the path is heated. The rate of flow of the charge or relatively untreated hydrocarbon is continuously measured by therate of flow meter, or differential pressure responsive device 3, while a difierential pressure responsive device 4 is located with reference to the conduit l'beyond the heating means or after the flowing fluid has been subjected to heating or other processing.

The float actuated meter 3 is sensitive to the difierential pressure across an obstruction, such as an orifice, flow nozzle, Venturl tube, or the like, positioned in the conduit for eflecting a temporary increase in the velocity of the flowing fluid. Such an orifice may be inserted in the conduit between flanges as at 5. The meter 3 is connected by pipes 6, 1 to opposite sides of the orifice 5 and comprises a liquid sealed U- tube, in one leg of which is a float operativelyconnected to position an indicator 8 relative to where Q=MVW (1) where g=acceleration of gravity=32.17- ft. per sec. per

h=difierential head in it. of t e flowing'fluid.

The coeii'icient of discharge remains substantially constant for any one ratio of orifices diameter to pip diameter, regardless of the density or specific volume of the fluid being measured.

With C, M, an d \/2 g all remaining constant, then Q varies as \/h. Thus it will be seen that the float rise of the meters 3, 4 is independent of variation in density or specific volume of the fluid at the two points of measurement andthat the reading on the indexes 9, I I of diflerential head is directly indicative of volume flow. It the conduit size and orifice hole size are the same at both meter locations, then the relation of meter readings is indicative of the relation of densities and specific volumes.

This may readily be seen, for if it were desired to measure the flowing fluid in units of weight, Equation 1 becomes:

- W==CM vzq'na' (2) W=rate of flow in pounds per sec.

d=density in pounds per cu. ft. of flowing fluid.

h=difierential head in inches of a standard liquid such as water.

M=meter constant now including a correctionto bring it of Equation 1 into terms oi. h of Equation 2. 1

Assuming the same weight rate of flow passing successively through two similar spaced orifices 5, I2- and with a change in density as caused by the heating means 2, then the density at the second orifice I2 may be determined as follows:

Thus it will be ,observed that, knowing the density of thev fluid passing the oriflce 5 I may readily determine the density of the fluid passing the orifice I2, from the relation of differential pressures indicated by the meters 2, l.

After the fluid has passed through the orifice I: it passes through a further heating section oi.

th fluid in the section 22 and accomplish this through an interrelation of the difl'erential pressures produced by the same weight flow passing successively through the orifices 5, I2, I3.

The same total weight or fluid must pass through the three orifices in succession so long as there is no addition to or diversionirom the path intermediate the orifices. It is equally apparent that in the heating of a petroleum hydrocarbon, as-by the section 20 between the orifices 5 and I2, there will be a change in density of the fluid between the two orifices, and furthermore that an additional heating of the fluid, as by the section 22, will further vary the density of the fluid as at the orifice I3 relative to the orifice I2. Assume now that the conduit I is of a uniform size throughout and that the orifices 5, I2 and I3 are of uniform diameter and coeflicient or characteristic. "Through the agency of the meter I6 the difl'erential pressure existing across the orifice I3 is continuously indicated upon an index I8 by indicator IT. The mean density of the conversion section 22 is then obtained by averaging the density of the fluid at the orifices I 2, I3. As for example: v

The density of fluid at the orifice I3 may be obtained in the same manner, relative to. the

7 density of the fluid at the orifice 5, as was precific gravity of the fluid entering the system) may be directly computed from the readings of the indexes 9, II, I8. This is of course on the basis that the orifices 5, I2, I3 are the same, and that the capacity of the float meters 3, 4, I6 is the same.

If .the meter 3 is on a weight rate basis and v indicates in terms of W=#/hr. then W I el -K where K=a constant, and

In this event it is not necessary to determine the density or specific gravity of the fluid entering the system, as at the orifice 5, unless it departs from that to whichthe flow meter is calibrated, in which case the meter reading must necessarily be corrected to design condition.

Now as the specific volume increases progressively from locations 5 to I2 to I3 the difierential pressure across these orifices increases in like manner, and in the operation of such a cracking still it may be that the differential pressure across an orifice I3 will be several times that'across the orifice 5 if the orifipe sizes are equal. I have, therefore, indicated that these orifices may be of an adjustable type whereby the ratio of orifice hole to pipe area may be readily varied externally of the conduit through suitable hand wheel or other means. The actual orifice design in terms D=diameter of equivalent circular orificehole in inches I c=coefllcient of discharge f=factor of approach sp. vol.=cu. it./lb.

Now considering that orifice I2 is so adjusted that its cfD is different from that of orifice 5, we may then determine the densityat I2A as follows: 1

d1n=CR where In similar manner I may determine the density at the orifice l3 regardless of the orifice area, so long as I take into account the cfD oi the orifice in the above manner. It will thus be seen that if the specific volume of the flowing fluid increases so rapidly that the differential heads at successive orifice locations (for the same design orifice) become many times the value of the differential head at the initial orifice, it would be impractical to attempt to indicate or'record such differential heads relative to a single index or record chart without one or more of the indications or records going beyond the capacityof the index or chart. There are two ready means of overcoming this practical difliculty, the first being an adjustment of the successive orifices, such as l2, l3 to have new values of eyD such that the indicator or recording pen will be kept on the chart; and the second through varying the basic capacity or the meter 4 or 16 relative to the meter 3. This latter method comprising so arranging the meter 4,

for example, that it requires 50% greater differ ential pressure to move the related pointer over full index range than in the case of meter 3. This may readily be accomplished by properly proportionlng the two legs of the mercury U-tube, on one of which the fioat iscarried: Of course it will be necessary to take such change in capacity into account when utilizing the difierential head readings in determining density or mean density.

For example, the reading of the pointer relative to the index should be on a percentage basis of whatever maximum head the meter is designed for. Then the total head corresponding to the indicator reading will be available orithe proper correction may be applied. Assume that the meter U-tube 3 is so shaped that it requires l' waterdifierential applied thereto to move the indicator 3 from 0 to 100% travel over the index 9, and that for meters 4 and I 6 it requires 250" water difier ntial to cause the indicator l0- to move from 0 to-100% over the index II, and i1 relative to l3. Then:

F 'a=% float travel of meter: 3 F4'=% float travel of meter 4 substituting in (7) dt=d.(

and

The fluid after passing the orifice 5 enters the heating section 20, having a hand actuated regulating means, 2|. The fluid then passes the orifice I2 and enters the heating section 22 wherein the heating is regulated by a.control device 23. I have shown herein in. diagrammatic fashion that thevalues'hsand hm are applied to a mech. nism 24, and the values-ht and hill are applied to a mechanism 25. The resultant value of density of the fiuid at the orifice l2 from the mecha-' nism 24, and the resultant. value oidensity of the fluidat the orifice l3 from themechanism 25, are applied-to a mechanism 26 which indicatesby the pointer 21 upon the index 23 the value of mean density or the .fluid passing through the heater 22. Mean density and ha are then applied to a mechanism 29 from which is indicated a resultant, in terms of time by a pointer39 upon an index 3 I.

In the operation of such a cracking still it is of considerable importance to determine the time-temperature relation of the conversion section, for example, the time that any particle re-.

mains in this section and the temperature to which it is subjected. To determine such temperature I indicate at 32 the bulb of a gas-filled thermometer system of which 33 indicates the connecting capillary and 34 a Bourdon tube whose free end is positioned responsive to the temperature at the bulb location.

The temperature sensitive means 34 and the time indicating means-30 then act through a mechanism 35 to move an indicator 36 relative to an index 31 to indicate directly the time-temperature, relation of the fluid through the heating section 22.

I have indicated that the control mechanism 23 may be positioned in accordance with meandensity, time, or time-temperature relation. To accomplish this I provid air pilot valves 38, 33.

- 40 positioned respectively by the indicators 21,

30, 33 for controlling a pressure fluid and selectively made effective upon the control mechanism 23 by means'of the valves 4|.

The air loading pressure from the pilot valves 33, 39, 40 may be selectively made effective upon a fluid flow control valve in the conduit I through the agency of hand valves A and the pressure which I preferably employ to accomplish the resultswhich I have just described as diagram- .matically'illustrated in Fig. 1. For instance it the associated rotor.

will be observed that according to Equation 5 it is necessary in determining the mean density 01' the conversion section to obtain the ratio of the differential heads at orifices 5 and It; then to obtain the ratio of the difierentiai heads at orifiees 5 and II, to then average these ratios. My

method is based on the use oi logarithms, a process well known in mathematics, whereby it is possible'to obtain a quotient by subtraction ora product by addition. In connection with logarithmically designed cams I employ sell-synchronous motors which lend themselves readily to. addition or subtraction through differential windings, as well as having the feature of ready grouping at remote locations.

- I indicate such self-synchronous generators for transmission of position at 42, 42A, 43, 44, 4|, 4' and 41, while the self-synchronous receiving mo-' tors are indicated at 48, 49, 50, Ill2,-53-54, 55-56, 51 and 58. The transmitting generator in each case is operated at a suitable angular rotation through the angular positioning of the rotor or single phase field winding. The stator or armature is in each case provided with a 3- phase winding. The field windings of each transmitting generator are energized from a suitable source of alternating current supply.

The operation or systems or this general character for thetransmission of angular tnovement is well known in the art, Voltages are induced in the 3-phase stator windings of the transmitter or receiver by the single phase field winding on When the rotor oi' one oi the transmitters is moved from a predetermined position with respect to its stator, a change is effected in induced voltage in the armature winding and the rotor of the receiving motor assumes a position of equilibrium relative to the transmitting generator, wherein the induced voltages in the S-phase windings are equal and opposite.

and consequently no current is set up in the armature winding. If the rotor of one of the generators is turned and held in a new position the voltage is no longer counterbalanced, where by' equalizing currents are caused to flow in the armature windings which exert a torque on the rotor of the receiving motor, causing it to take up a position corresponding to the position of the transmitted generator.

The receiving motors 48, 49, 50 are individually positioned in synehronism with the transmitting generators 42, 43, 44. Between the indicator arm 8 and the transmitting generator 42 I interpose a cam 59 having a rise proportional to the logarithm of its angular motion to the end that the receiving motor 48 and the recording indicator I positioned thereby assume a position corresponding to log hs. Similarly the indicator arm 6| is positioned by the receiving motor 49 in accordance with the value of log hi2, while the indicator 62 is positioned in accordance with the value or log hi3.

Actually the design is such that the transmitting generator 42 (positioned in accordance with log F3) attains maximum desired rotation with from 10-100% full float travel. No motion of the I generator 42 occurs when the float oi. the meter 3 moves over -1 0% of its travel range, This because it is impossible to have a logarithmic. cam start at zero, as the number 0 has no logarithm. Also because the logarithmic character- 'istics are such that I'would have as much cam rise for from 1% to 10% of float rise as for from 10% to 100%. Thus I may make the cam 59,

. and the similar cams of the meters 4 and I. of

assume practical size and proportion by sacrificing only the first 10% oi the'fioat travel or the meters and with the expectation that the-operation will not normally be below 10%0! full float travel.

In addition to indicating and recording in interrelation upon the record chart 63 the values or the log oifthe diiterential pressures at the three orifices, the positions oi! the transmitting generators 42', 43, 44 are utilized through the agency or diilerential self-synchronous devices to algebraically add the value of the log it for the dinerentorlfiees and thus accomplish the ratio operation. Angular movement imparted mechanically to the rotors of the transmitting generators 42,

a: will result in anangular positioning of the rotor of the receiving motor Sl-SI. Similar action occurs between the transmitting generators 42, 44 and the receiving motor 53-44; and between the transmitting generators 42A, 46 and the receiving motor -56. 1

The receiving motors Sl-II, 53-54, and 55-56 have 3-phase rotor windings and 3-phase stator windings and are commonly known as difi'erential sell-synchronous motors, for in-each case they are responsive to two of the transmitting generators and assume a rotor position corresponding in diiferential effect from the two related transmitters. For example, the receiving 'motor 5l-52 has its rotor positioned responsive to a differential between the position of the rotor 42 and that 01' the rotor 43, or according to log his-Flog 7m, thus performing the mathemati operation:

has its rotor positioned responsive to a difi'erential between the pomtion oi the rotor 42 and that of the rotor 44, thus performing the mathematical operation:

- by the rotor of 5l--52 will indicate relative tothe index 64 the instantaneous value of log d1: while on the index 65 may be read the instantaneous value or log due.

The rotor of fol-52 anguiarly moves a cam 06 having a riseproportional to the antilog of its angular motion; likewise the rotor 01'53-54 angularly moves an antilog cam 61. Thus the vertical movement of a roller at the lower end of a link 68, riding on the cam 68, is proportional to (11: and that of 69110 (113.

To obtain the mean density through the conversion section IE it becomes necessary to solve Equation 4 and this I accomplish through a differential mechanism" adapted to position an indicator 'I'I relative to an index and recordin chart Iii to continuously record thereon the value of M15.

It is to be understood that if the basic capacity of meters 3, 4, l6 vary one from the other, then as previously brought out, this may be taken care or train (8). eThelinkage-through which the 'arm'l'll' positions" and the linkage through which log-giorloggf; will position the arm 12 relative tolthe index I4 according to d5 Jaye Qf ir i2 or an. Likewise on may be indicated'dis.

The diflerential I6 then'positions the arm 'l'l according to is-H 1:

ormdis. I

At 19 1 indicate a manual adjustment of the motion or arm 11 to take into account deviations in value 01 ds of (9) from design conditions, as might be attributed to changes in specific gravity, temperature, etc.

The arm 11 is adapted to position a logarithmic cam 19A for moving a transmitter 48 proportional to log man. The meter 3 positions a cam 59A formo ving a transmitter 42A proportional to log /h5, which so long as de remains constant equals log W where W is rate of flow in lbs. The difierential motor 55-58 isthen under the influence of the transmitters 42A, 48 representative 01' log W and log mdis and the resulting angular motion of cam 88 is:

log T=logmdis-log W Cam 88 is of antilog design and the arm 8l is moved relative to record 82 to indicate the time of detention 01' any particle of fluid in the heating section llhirom:

vin u where T=Time any particle is in section l8. V=Volume between A and ISA (011. it.)

, mdis=Mean density (lbs. per cu. it.)

W==Rlate of flow (lbs. per unit T) The position of the arm 8| is used to angularly position a transmitter 45, in turn positioning a.v

receiver 51 and cam 83. Closely related is a cam positioned by a receiver 58 under the control of a transmitter 41 responsive to mean temperature of. the fluid mixture. Temperature responsive bulb. 86 is located in the fluid at the outlet of the heating section 15, while bulb .81 is located at the inlet to the section. Bourdon tubes 88, 89 are arranged to position the transmitter 41 according to the mean temperature of the fluid through the section l5. The cams 83; 84 may be designed as uniform rise cams or to take care of any characteristics. or relationship as may .be desired. Through their interrelation. anindicator 851s continuously positioned relative to an index and recording chart 88 to The corresponding conversion section II.

An indicator pen so is positioned with the indicator as by time-temperature relation but is further providecPwith a stockiactor adjustment 9| so that the pen 88 recordson the chart 88 the yield per pass. The stock factor adjustment 9| is available to correct for deviations in specific gravity, anilin number, and such other variables as may afl'ect the charge .or fluid entering the e conduit I.

The oriflce 12 may be within the heater having a fluid flow path. In Fig. 2 the orifice I2 is shown away from the coils l4, l5 and heat source 2 only as a matter or clarity in the drawings.

In Fig. 3 I illustrate a further arrangement to indicate or record time of detention or treatment. A rate-of-flow meter 3A is of a type having a shaped liquid sealed bell adaptedto correct for the quadratic relation between differential head and rate or flow and positions a cam I80 directly in accordance with W orpounds per unit of time. The transmitter llll moves proportional to log W.

The differential receiver l0 2l83 is sensitive to log W and log his positioning the antil'og cam I84 according to L I v10g W-1og b -log hm Likewise thereceiver "Ii-I86 is sensitive to log W and log hi3 positioningthe antilog cam I01 according to The pointer I08 then indicates relative to the dicates and records time of detention or treatment, from:

- a i an X W]; We idm d g (15% X V=Volume (a constant) W KEV hsdg While I have chosen to illustrate and describe the functioning. of my invention in connection with the heating of petroleum or hydrocarbon advise the timetemperature relationship for the oil, it is to be understood that the apparatus is equally applicable to the treatment, processing,

or working of other fluids, such for example, as in the vaporization of water to form steam.

Referring now in particular to Fig. 4, I illus-' trate thereon an arrangement of my invention in connection with a common type of petroleum or other heater. In this arrangement the fluid flow path constitutes a plurality of parallel long smallboretub'es, and I have illustrated in Fig. 4 that two tubes pass in parallel through the furnace. It is equally possible that there may be three or more tubes in parallel within the confines of the furnace.

The supply fluid or charging stock enters by way of the conduit I26, passing through a regulating valve [M to a Y fitting I22 containing hand valves I23, I24 in its branches. From the Y fitting I22 the two parallel conduits I25, I26 pass through the furnace chamber either sinously or coiled or arranged as may be best desired for heat transfer purposes, as well as case of support and other constructional details.

The conduits I25, I26 leave the heater and join together by a Y fitting I21, thence passing to an outgoing conduit I26. I

.A common source of heating (the burner 2) is shown for heating the parallel tubes I25, I26; although thearrangement of heating may be in any of various well known manners or types of apparatus; The supply of fuel for heating, or of the elements of combustion, isunder the control I of a regulating valve I 29.

In the operation of such a fluid heater it is of primary importance that the product leaving the tubes I25,..I26 shall ..be insubstantially the same condition as to density or other quality, and furdensity of the fluid leaving the separate conduits I25, I26. 1 I 1 The density determining devices I32, I35 are shown in Fig. 4 diagrammatically as connected to a device I36 incorporating mechanism previously explained for either averaging the two densities or obtaining the ratio of the two. In con-v tively applied, by means or the valves I39, I46

upon either the fuel control valve I29 or' the rate of charge control valve I2 I.

In similar manner the device I36 positions an I air pilot valve I39 establishing an air loading pressure representative of the ratio of the densities, and which is made effective selectively through the hand control valves I, I42 upon either the fuel control valve I29 or the rate of charge control valve I2I.

It will thus be seen that I may selectively con-- trol the treatment, 1. e. the rate of charge and/or the rate of firing of the furnace, either from the average of the densities of the fluids leaving the conduits I25, I26 or selectively from the ratio of said densities. I may control either the supply of the elements of combustion or the rate of supply of fluid to'the conduits from either the average or the ratio of the densities; or I may control one from one and one from the other.

. throughthe conduits I25,..l26'I intend to mean therrnore that the condition or quality of the total fluid passing out of the conduit I 26 shall be as desired. Thus it is necessary to control either the rate of charge by means of the regulating To accomplish this purpose, I preferably utilize the apparatus previously discussed in connection with the other flgures of the drawings. '1 preferably ascertain the density in situ of the fluid leaving the conduit I25 separately and independently from a determination of the density of the fluid leaving the conduit I26. I then obtain either an average of the two densities or a'ratio of the two densities and selectively from either the average or the ratio I control either the rate of fluid ibnizfiw to the circuit or the rate of heating, or

The density or speciflc gravity of the charge fluid-entering the conduits I25, I26 is known and uniform. The meters I39, I3I located at the inlet and at the outlet of the conduit I25 are similar to the -meters 3 and 4 previously described, and are each adapted to position a transmitting self-synchronous generator. The two transmitters are electrically connected'to the device I32 wherein is performed the determination of density of the fluid leaving the conduit I25,

as previously explained in connection with Fig. 2.

In similar manner the meters I33. I 34 are connected at the inlet and outlet respectively of the either a heating or other acting upon which will vary a condition of the fluid, such for example as density. The treatment may broadly be a variation or control of the heating as well as of the rate of flow of the fluid through ,the treatment zone.

I have not felt it necessary to. duplicate in Fig.

4 the detailed arrangement of averaging and ratio means which has already been explained in connection with other figures of the drawings.

While I have illustrated and, described certain preferred embodiments of my invention it will'be understood that I am to be limited thereby only as to the claims. tinuation-in-part of my copending application Serial No. 152,855 filed July 9, 1937.

What I claim as new, and desire to secure by Letters Patent of the United States, is: a

'1. In a fluid heater through which a fluid is continually passed under pressure without change in weight rate of flow while being heated, ap-

paratus adapted to continuously determine the in situ density of the fluid. after it has been heated, characterized by, means in the flow path at the entrance to the heater and after the heating separately measuring the velocity of fluid flow. a transmitting generator associated with each of said means, means for positioning said generators in relation to logarithmic functions of said measurements, a "receiving motor positioned in accordance with the difference in positions of the generators, and means positioned by the said receiving motor in antilogarithmic relation to the position of the motor.

2. Apparatus for determining the mean density of a flowingfluid throughout a section of its flow path, comprising in combination, means including a member positioned in accordance with This application forms a c0nfunction oi. the said second receiving motor, and an indicating element positioned jointly by said last two named means.

and third members for obtaining the antilogarithm of the logarithm of the quotient of said functions of the reference and inlet flows, other means under the control of said second and third members for obtaining the antilogarithm of the 4; In a fluid heater having a fluid path, means for determining the density of the fluid at a point in the path within said heater,- comprising in combinatiommeans in said path at the entrance to the heater and at said point to measure the velocity of flow, a transmitting generator associated with each of said. last named means, means for positioning 'said generators in relation to logarithmic functions of said measurements, a

logarithm of the quotient of said functions of the reference and outlet flows, and means positioned by said means and said other meansfor continuously averaging the two antilogarithms.

3. In afluid' heater having a fluid path, means for determining the mean density of the fluid between two points in the fluid path comprising in combination, means in said fluid path at the entrance to the heater and at each of the two points to measure the velocity of flow, a transmitting generator associated with each of said means, means for positioning said generators in relation to a logarithmic function of said measurements, 9. first receiving motor positioned in accordance with the difference in positions of the generators associated with the measuring means responsive to the velocity of flow at the entrance to said heater and at one of said points, means positioned by the first receiving motor in relation toan antilogarithmic function of the position of the first receiving motor, a second receiving mo- I tor positioned in accordance with the difference at the entrance to said heater and at the other of said points, means positioned by the second receiving motor positioned in accordance with the diflerence in positions of the generators associated with the measuring means responsive to the velocity of flow at the entrance to said heater and at said point, means positioned by the re- \ceiving motor in'antilogarithmic relation to the flow path while being subjected to a. condition change which will afiect the density of the fluid without changing its weight rate of flow the imtermination and the flow factor after condition change, and means continuously algebraically adding the ratios so obtained.

' JOHN F. LUHRS.

. 7 receiving motor in relation to an antilogarithmic 

